Trunked radio systems have developed in response to an ever increasing demand for radio communications services. Previous radio communications systems have expanded to the point where potential users are denied service because the available frequency resource is being utilized. Typically, subscribers using older mobile radio system were constrained to operate on a single frequency, thus limiting the number of users which could be expected to be assigned to that frequency. Trunked radio systems alleviate this inherent spectrum limitation by actively assigning operating frequencies to mobile radios based on channel availability. In other words, the mobile radios used as subscriber units in trunked radio systems are operative on a number of frequencies which are assigned to the subscriber unit 102 based on channel availability.
A prior trunked radio system is shown and described in U.S. Pat. No. 4,012,597, entitled "Transmission Trunked Multichannel Dispatch System with Priority Queuing", by Lync et al., filed Nov. 24, 1975 and assigned to Motorola, Inc., Schaumburg, Ill. The trunked radio system described in this patent is referred to as a dispatch trunked radio system. In a dispatch trunked radio system, a mobile radio establishes communication on the system by transmitting a channel request to a system central controller on a predetermined inbound frequency. The central control unit responds to the subscriber unit with a signalling word which authorizes the subscriber unit to operate on a specific communication channel. Typically, the channel request transmitted from the subscriber unit includes information which identifies the subscriber unit to the central control unit. Additionally, the subscriber unit responds to commands sent from the central control unit based on radio I.D. information included with outbound channel assignment information. Prior dispatch trunked radio systems developed around a basic unifying protocol which communicates radio I.D. and channel assignment information. However, these systems are inherently limited in the type of features which may be implemented because of the limited amount of information which is communicated between the central control unit and the mobile radios of the dispatch trunked radio system.